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What experiments prove the greenhouse effect?

I am seeking for a proof that CO2 is a greenhouse gas. I posted this on Skeptic.SE recently but found no help in seeking for proof:

I assisted to a physicist conference in my university a few years ago against the case that carbon dioxide was a cause of global warming. The main point was that CO2 is not a greenhouse gas. I did a research to find evidence for either side and found absolutely nothing.

So, is carbon dioxide a greenhouse gas? If yes, has it been demonstrated in a scientific paper?

Here are some discussion articles describing arguments against CO2 being a greenhouse gas:

Home experiments

Ph.D. Tim Ball

Evidence from temperature rise through history

I understand that the sources do not prove that CO2 isn't a greenhouse gas...but I can't find evidence proving the contrary. Since a real physicist with many years of experience a demontrated to me that it is not possible by definition, I need to find proof to accept the other premise.

I found this post on physics.SE, but the asker do not seek proofs and simply accept what the others are saying.

Baically, to have a greenhouse effect, a gas needs to absorb only a portion of a wavelength and let pass another portion. Since CO2 absorbs some IR, a part of it is remitted to earth. The spectrum is absorbed by the earth (everything) and it is remitted on another wavelength which allows a different proportion to pass through the CO2 layer. The question is, why the heat would be remitted toward earth (it should go in the colder direction)? And also, if CO2 is all around the earth and blocking a certain wavelength, why would increasing it warm the earth? Am I missing something?

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  • $\begingroup$ This is better suited for chemistry.stackexchange.com $\endgroup$
    – Siyuan Ren
    Commented Sep 4, 2012 at 0:32
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    $\begingroup$ I believe my own answer to the previous question answers this question as well. Claiming that CO2 is a greenhouse gas equates to claiming that a higher concentration (in current conditions) causes positive radiative forcing. This is scientifically convincing beyond a shadow of a doubt and you can find those associated arguments and evidence articulated in Physics SE in other questions. $\endgroup$
    – Alan Rominger
    Commented Sep 4, 2012 at 1:37
  • $\begingroup$ @AlanSE, I am mostly satisfied with your answer. I would have hoped for a scientific publication confirming your saying...but I heard from many physicists that it is so trivial that none bothered publishing results. I will vote for the closing of this topic (physics.SE do not seek to provides scientific publications, so my request would be offtopic) $\endgroup$
    – Zonata
    Commented Sep 4, 2012 at 3:21

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First of all the very name is misleading, as real green houses get heated by obstructing convection and not by the gases in the green house.

The question can be reformed as : does the existence of CO2 in the atmosphere contribute to the heat retention capacity of the atmosphere which is what the "greenhouse effect" is about.

The answer is yes, it has been measured and it is small . In this image one can see the gases which can absorb the incoming solar radiation, and the main contribution is from H2O, water vapor. The CO2 contribution, though small,is not negligible.

These are old measurements as you can see from the reference on the figure.

So the answer is yes to CO2 being a "greenhouse gas" among others. Now the importance given by the community of climate change advocates to CO2 is completely dependent on computer modelling of the earth's climate. The importance comes from an assumed in the programs feedback process, where a small increase in atmospheric CO2 induces a large increase in atmospheric H2O in a run-a-way mode. This is another story though.

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  • $\begingroup$ This intro to the "greenhouse effect" seems fairly good : bouman.chem.georgetown.edu/S02/lect23/… . $\endgroup$
    – anna v
    Commented Sep 4, 2012 at 3:57
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    $\begingroup$ I'm unsure of the wording "run-a-way" applied to the water vapor feedback. To be as literal as possible, water vapor feedback predicts a greater temperature change is required for the natural emission mechanism to adjust to a heat flow imbalance. There are other phenomenon predicted by scientists that I would unambiguously call run-a-way, like the potential release of ocean floor methane hydrates. $\endgroup$
    – Alan Rominger
    Commented Sep 4, 2012 at 4:13
  • $\begingroup$ @AlanSE When a feedback is positive then it will run away, it is an unstable situation ( think loudspeakers) until it hits some limit of course (the power available in loudspeakers), I believe in the models used by the IPCC no such restraints are introduced when they predict those huge catastrophic numbers of sea rise and temperature rise, which by the way have not materialized yet, when one looks at the ten year old predictions. $\endgroup$
    – anna v
    Commented Sep 4, 2012 at 5:07
  • $\begingroup$ A good answer, but spoiled by the mistakes in the last paragraph. $\endgroup$
    – 410 gone
    Commented Sep 4, 2012 at 6:08
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The wikipedia page on Arrhenius (http://en.wikipedia.org/wiki/Svante_Arrhenius#Greenhouse_effect) has some information. I'm sure you can find a lot more with about 10 seconds of googling, unless your real interest in posing the question is polemical, rather than information seeking.

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  • $\begingroup$ I googled a lot, believe me. I found no serious evidence (scholarly book, scientific publication, etc.). I only find childish explanations. I am not seeking polemic, I just seek for science proofs. I am sorry if you misinterpret my intentions. $\endgroup$
    – Zonata
    Commented Sep 4, 2012 at 2:59

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